A Storm-Chaser Who's Looked Straight Into a Tornado's Heart

The tornadoes that recently swept through the Dallas Forth Worth area were a reminder of their destructive power. While the most sensible response might be to go as far away as possible from such things, atmospheric scientist Joshua Wurman runs right at them.

Wurman in Battle Pass, Wyoming, in November. Behind him is the Doppler on Wheels, the mobile radar truck he invented.

photographs by Beth Wald

By early June of 2009, Joshua Wurman was exhausted and discouraged. For weeks, the nomadic teams of VORTEX2 (the second Verification of the Origins of Rotation in Tornadoes Experiment) had crisscrossed the Midwest in pursuit of the violent thunderstorms that can generate tornadoes. Yet all that he and the other meteorologists had encountered so far were a few rain showers. With more than 100 participants, 11 radar trucks, 13 instrument-laden vehicles, an unmanned plane, and millions of dollars from the National Science Foundation in play, the most ambitious tornado field study ever was at risk of failing. The weather was just too nice.

But when the scientists finally intercepted their first tornado of the season in Goshen County, Wyoming, it offered an amazing coup. For the first time, they were able to capture detailed data on the entire life cycle of a tornado, from gestation to birth to demise. Analysis of information from this storm and dozens of lesser intercepts in 2009 and 2010, combined with new insights from computer simulations, may finally answer the researchers’ biggest question: What triggers a tornado? Zeroing in on how tornadoes get going could lengthen warning times from the current, dangerously short average of 13 minutes and also lower the rate of false alarms. DISCOVER recently spoke with Wurman, who has probably collected data on more tornadoes than any other scientist, about his theory of how tornadoes form, the twisters that claimed 548 lives in 2011, and a recent storm that flat-out awed him.

What makes tornadoes so unpredictable? We know the fundamentals of how supercell thunderstorms—the ones that produce tornadoes—form. We know that there need to be certain conditions of temperature, relative humidity, and wind speeds at different altitudes. What we don’t really understand very well is why only 25 percent of the supercells make tornadoes and when in their life cycle they do it: Why did that particular supercell make a tornado now, not 15 minutes ago, or 15 minutes from now? The reason we drive 15,000 miles a year to catch 10 tornadoes is because we don’t know which supercells are going to make them or when.

Why is it so difficult to collect data on a tornado? It’s a pretty foggy, blurry view—we’re looking through a distorted window with cracks in it. We do fairly well at seeing the winds throughout the storm. Radar is great at doing that. We should probably get a B+. Where we get an F, maybe an F+, is in measuring the temperatures and relative humidities, what we call the thermodynamics, inside the storms. We know that something is causing the winds to move up and down in the supercell, and we believe that the temperatures and relative humidity are critical to that process. Yet we have almost no direct way of obtaining those numbers. We tried with unmanned aerial vehicles but didn’t get many measurements.

Why was capturing an hour’s worth of data on the Goshen tornado such a big deal? We observed the tornado from well before its genesis through its dissipation, using different radars and instrumented vehicles. So we were able to capture its whole life cycle. It’s the best data set ever. We think we have at least a smoking gun, if not the smoking gun, for why this particular storm made a tornado when it did, and we think there’s a good argument for the case that a secondary surge was causing it.

Joshua Wurman [right] confers with technician Justin Walker, another
member of the VORTEX2 team, inside the Doppler on Wheels.

photographs by Beth Wald

What does that tell us about how thunderstorms make tornadoes? We’ve known for decades that all supercell thunderstorms have a gust front, which is the boundary between the moist, warm air that is flowing into the storm and the generally cooler air coming down out of the storm. But what we noticed in several cases recently is that thunderstorms that are making, or are about to make, tornadoes, have a secondary front, which is like a second wave of air rushing down from aloft. A strong downdraft has an important function: It brings the rotation to the ground. But for a tornado to form, you still need to tilt the rotation into the vertical, and this requires a nearby updraft. The intensity of the downdrafts and updrafts is vital, because in the end there needs to be a lot of stretching, which is when you take that existing rotation and turn it into something really violent like a tornado. It’s like a figure skater pulling in her arms and spinning faster and faster.

In the Goshen County tornado, we have a strong suspicion that the development of this secondary surge or front sparked the genesis of the tornado. We need to test this. If, after looking at more cases, we can demonstrate a causal link, then perhaps in the future a forecaster observing the development of a secondary surge will have an increased ability to forecast tornadogenesis.

The data analysis emerging from VORTEX2 also identifies another possible trigger, a “descending reflectivity core.” What is that, and how does it work?Some supercell thunderstorms have a descending core of intense rain and hail wrapping around the west side of the storm. That’s what we call a descending reflectivity core, or DRC. This DRC drags rotating air downward from maybe four or five kilometers up and might cool the air in various places. As you drag the air downward, you create rotation and antirotation in different parts of the storm, and that seems to occur around the time of tornadogenesis. Right now these two features, the DRC and the secondary surge, hold the most hope for explaining why some supercells are able to generate rotation near the ground and why the low-level rotation is turning into a tornado when it does.

Why was 2011 the deadliest tornado season we’ve seen in 75 years? Were the storms stronger than usual last year? In recent years, we’ve become very used to tornadoes causing a relatively small number of deaths. A few dozen is typical. Unfortunately, while some of that may be due to better forecasts, some of it is also due to luck. Last year, the tornadoes hit larger places. They hit Tuscaloosa, they hit Joplin. The total number of tornadoes may have reached 1,800, which is exceptional, but the big spike in deaths was really based on a few individual points. Just one tornado in Joplin killed almost three times the yearly average of the last few decades. The Joplin tornado was rated EF5, but there wasn’t some added degree of destruction. The difference between Greensburg, Kansas [where an EF5 tornado killed 11 people in 2007], and Joplin was how many people got hit, not the strength of the tornado.

You were busy sifting through data from VORTEX2 in 2011. Was it frustrating to sit out such a volatile tornado season?We did go out a couple of times. One day we got some fascinating data in a strong tornado in Oklahoma that had winds of about 200 miles per hour. We observed this tornado as it crossed a lake, and in the radar we saw this very clear central eye and a very strange wind, because it was lifting up a huge amount of water. We saw for the first time ever, I think, a tornado surge, like a hurricane surge. Then, as the tornado made landfall—and that’s a term we usually use with hurricanes—it just started shredding the forest. The eye suddenly filled up with a big ball of debris. From a scientific perspective, it’s very interesting because one of the great limitations we have in meteorology is that we’re not a laboratory science. But in this case, we had a tornado that was experiencing pure, simple conditions: lake for a few minutes and then pretty simply, woods. The structure of the tornado changes dramatically right when it crosses from the lake to the forest.

You have warned that the risk of a tornado-caused catastrophe in this country is underestimated, or even overlooked altogether. My colleagues and I wrote a paper in 2007 (pdf) that asked, what if one of those large tornadoes that we’ve observed with the Doppler on Wheels went through the suburbs of Chicago or St. Louis? This is a worst-case scenario, but I’d say it was a plausible worst-case scenario. Tens of thousands, even 100,000 homes could be destroyed. I think we should have at least some degree of preparation.